1. Field of the Invention
The present invention relates to a permanent magnet for use in an environment in which the magnet is exposed to a radiation at an absorbed dose of 3,000 Gy or more. More particularly, the present invention relates to a permanent magnet for a particle accelerator, which may be used in either a synchrotron for the purpose of physical properties research or a cyclotron in the field of radiotherapy. The present invention also relates to a magnetic field generator including a plurality of such magnets.
2. Description of the Related Art
Examples of particle accelerators includes a synchrotron, which is used to generate a high-energy particle beam for the purpose of physical properties research, and a small-sized cyclotron, which produces a radioisotope for use in the diagnosis of cancer. Recently, those accelerators have just been introduced into a sort of radiotherapy for directly irradiating the diseased part of a cancer sufferer with a proton ray, not just for those diagnosis purposes.
A particle accelerator includes a mechanism for receiving an incoming particle beam, a mechanism for accelerating charged particles with the application of a radio frequency electric field thereto, and a mechanism for applying a magnetic field to bend the particle beam in any desired direction.
In a synchrotron, for example, a bending magnetic field for making the particle beam travel along an annular orbit that is called a “main ring” or a “storage ring”, a focusing magnetic field for focusing the particle beam in the orbit, and a bending magnetic field for making the particle beam incident on, or outgoing from, the main ring are used. In a cyclotron on the other hand, a uniform static magnetic field is used to accelerate the particle beam spirally.
In the prior art, the magnetic fields described above are generated by electromagnets in both the synchrotron and cyclotron.
In a particle accelerator, some parts thereof need to be adjusted by making the magnetic field strength variable like a focusing magnetic field according to the operating principle but other parts thereof need a constant static magnetic field during the operation. At the beam outlet port of the synchrotron, the orbit of the particle beam is slightly changed by applying a pulsed magnetic field from an electromagnet called a “kicker magnet” to the particle beam traveling along the main or storage ring. That particle beam is further bent significantly by an electromagnet called a “septum magnet”. Such a septum magnet needs to generate a strong and uniform static magnetic field and is provided near the main ring of the particle accelerator. Thus, the magnetic field leaking from the space in which the septum magnet generates the strong magnetic field (i.e., magnetic field generating space) to the external space needs to be minimized (e.g., to 5 mT or less).
As to the cyclotron, a uniform static magnetic field needs to be generated as described above. However, if protons are used as particles to accelerate, then a strong static magnetic field of 1.0 T or more needs to be generated to bend the protons because protons have greater mass than electrons.
Magnets for use in those particle accelerators are disclosed in Japanese Patent Application Laid-Open Publications Nos. 64-72502, 8-255726, 2001-28300 and 2003-305021, for example.
As described above, an electromagnet for use in a particle accelerator needs to generate a strong magnetic field. Thus, during the operation, a large amount of current needs to be supplied to the coil of the electromagnet.
However, when a large amount of current is supplied to the coil, a lot of Joule heat is generated by the coil. Accordingly, to remove this heat quickly, a cooling mechanism needs to be provided around the coil.
Furthermore, when an electromagnet is used, the strong magnetic field that is generated intermittently by the electromagnet easily does damage on respective members that form the electromagnet, which is also a problem. In addition, a huge quantity of yoke material for use to make the electromagnet is mainly composed of iron, and therefore, is easily radioactivated when exposed to the radiation generated from the beam line. As used herein, the “to radioactivate” refers to a phenomenon that a portion of a substance exposed to an accelerated particle beam is transformed into a radionuclide and comes to have radioactivity itself. If the yoke material is radioactivated, then it becomes difficult for workers to access the electromagnets for maintenance purposes.
To avoid these various problems to arise when electromagnets are used in a particle accelerator, the electromagnets may be replaced with permanent magnets. For example, in a storage ring for a particle accelerator developed by Fermi Laboratory, United States, hard ferrite magnets are adopted. However, hard ferrite cannot generate a strong bending magnetic field (e.g., of about 2 T) if its size remains small. Thus, it would be impossible to use small-sized particle accelerators in general hospitals extensively.
A 2-17 SmCo sintered magnet is a high-performance magnet, which is not demagnetized so much even when exposed to a radiation and which has a maximum energy product exceeding 240 kJ/m3. Thus, to generate a strong magnetic field for a particle accelerator, the 2-17 SmCo sintered magnet could be used. However, Co, which is a main ingredient and essential element of this magnet, is easily radioactivated. Thus, considering maintenance, it would also be difficult to adopt those magnets in the accelerator.
Meanwhile, an Nd—Fe—B based sintered magnet can exhibit as high performance as represented by its maximum energy product exceeding 320 kJ/m3, and therefore, can contribute to reducing the size of the accelerator effectively. In addition, the Nd—Fe—B based sintered magnet is less likely to be radioactivated than the 2-17 SmCo sintered magnet. Nevertheless, the Nd—Fe—B based magnet is easily demagnetized when exposed to a radiation.
Hereinafter, it will be described exactly how an Nd—Fe—B based sintered magnet is demagnetized due to exposure to a radiation. FIG. 1 is a schematic representation showing a portion of an Nd—Fe—B based sintered magnet on a larger scale. In FIG. 1, the open circles ∘ represent some constituent atoms of an Nd2Fe14B type crystal, while the smaller solid circle represents a radiation with energy E0 (a high-energy particle). This particle is supposed to fly along the arrow to collide against an atom located at the center of a region R. It should be noted that the radiation may be either a corpuscular radiation such a proton radiation, a neutron radiation, an alpha radiation, a beta radiation or a heavy ion corpuscular radiation or an electromagnetic wave such as a gamma ray or an X-ray.
As shown in FIG. 1, when the radiation collides with the nucleus of an atom in the Nd—Fe—B based sintered magnet, that atom may sometimes be shot away due to the impact of that collision but remains there in most cases. In the latter case, the incoming energy E0 is absorbed as heat into the magnet, thereby augmenting the lattice vibration around that atom where the collision has happened. As a result, the temperature rises locally around the region R. Supposing the temperature before the radiation energy E0 is absorbed into the region R is represented by TL and the temperature after the energy has been absorbed there by TH, the magnitude of the temperature increase is given by ΔT=TH−TL, which is proportional to the energy E0. If the temperature TH after the magnet has been exposed to the radiation exceeds the Curie temperature TC thereof, then the magnetization of the region R inverts during the cooling process irrespective of the coercivity HcJ of the magnet or the magnitude of the permeance coefficient Pc of the region R. The mechanism by which coercivity is produced in an Nd—Fe—B based sintered magnet is called a “nucleation type”. Accordingly, once magnetization has inverted in the region R, the overall crystal grain, including that region R, will eventually cause magnetization inversion. As the exposure dose of the radiation increases, such magnetization inversion is propagated to more and more regions (i.e., crystal grains) of the sintered magnet. Consequently, the overall sintered magnet is soon demagnetized significantly.
Once demagnetized in this manner, the magnet can no longer generate a strong magnetic field constantly. That is why no conventional magnetic field generator for a particle accelerator has ever used Nd—Fe—B based sintered magnets successfully enough to put it as a commercially viable product on the market.
In order to overcome the problems described above, a primary object of the present invention is to provide a permanent magnet for a particle accelerator and a magnetic field generator, in which Nd—Fe—B based magnets are used but are not demagnetized so easily even when exposed to a radiation.